I need other success actions. Is it possible to extend this part of the form? I've already added custom fields and customer save actions. I would like to redirect to a specific URL, not a node in the tree. Is this possible? I see the Success Mode list items under Meta Data but it doesn't look like they are associated with anything (assuming it's hard coded in the framework)

I've tried this myself but I ended up having to just create a save action to do a redirect. I think it's hardcoded in a way that you can't get to it. If you look at the template for Form, you see there are two fields, one for Success Message and another for Success Page. You can try to change those all you want but it doesn't affect anything.

Update: There is no way to do it that is nice. You would have to bring in tons of code from the sitecore libraries and make changes. The success is hard coded and expects a linkfield to work with even though the template can be a single-line text.

Use the save action or have them edit the template directly like the other answer suggests.